When a large, bulk-shaped oxide superconductor having a diameter of several tens of millimeters or more is solidified by slow-cooling from a partially molten state to promote crystal growth, for example, methods like that described below are generally employed. One example thereof is that a bulk-shaped precursor of an oxide superconductor is formed, and then it is supported by a rod-shaped material, a substrate or a sheet-shaped material which is placed below the precursor and is composed of a heat-resistant material such as Al2O3, YSZ (yttrium stabilized zirconia) or MgO. Alternatively, a method is employed in which a mixed powder of a superconducting powder having the composition of YBa2Cu3O7-x (so-called Y123 powder) and an oxide powder having the composition of Y2BaCuO5 (so-called Y211 powder), or a mixed powder of a superconducting powder having the composition of YbBa2Cu3O7-x (so-called Yb123 powder) and an oxide powder having the composition of Yb2BaCuO5 (so-called Yb211 powder), which has a peritectic temperature equal to a peritectic temperature of a precursor of a superconductor or lower, is placed on a metal mount or the like.
When a precursor of an oxide superconductor is placed directly on a support member such as a dish or crucible composed of a heat-resistant material such as platinum or Al2O3 (alumina) and heated to a temperature at which the precursor is partially molten, the precursor in the partially molten state reacts with materials comprised in the heat-resistant material and adheres to the dish or crucible. When the precursor is solidified, the precursor is subjected to large stress due to the difference in thermal expansion coefficient between the precursor and the dish or crucible, resulting in the formation of cracks which are undesirable for an oxide superconductor. The aforementioned general methods are used to avoid these problems.
Namely, in the aforementioned methods, the precursor in partially molten and solidified state is supported with a support member wherein the composition thereof is as close as possible to the composition of the oxide superconductor. As a result, stress loading caused by the difference in thermal expansion coefficient when the precursor is solidified, is reduced, and crack formation is prevented as much as possible in the oxide superconductor obtained with these methods.
In addition, as described in Japanese Unexamined Patent Application, First Publication No. Hei 5-229820, a technology is provided in which a precursor in a partially molten state is supported by floating on molten silver in a metal dish, and then the precursor is solidified from this partially molten state. Here, the silver hardly reacts with an oxide superconductor. It is described in the document that, due to the above characteristic, the resulting oxide superconductor can be easily removed from a solid of the silver after a melting and solidifying the precursor.
However, in the previously described methods using the rod-shaped material, a substrate or a mixed powder which consists of a material having a composition which is similar to the composition of the oxide superconductor, there was still the problem of susceptibility to formation of cracks by reason explained below, although the risk of crack formation is lower than the method using a support member such as a dish or crucible composed of platinum or Al2O3 (alumina).
Here, when considering the partially molten state of an oxide superconductor, a superconductor powder having the composition of YBa2Cu3O7-x decomposes at the peritectic temperature or higher as in the manner of the following formula (I).2YBa2Cu3O7-x (Y123)=Y2BaCuO5 (Y211)+L(3BaCuO2+2CuO)  (I)
In this formula (I), L represents a liquid phase. X represents the amount of oxygen deficiency in the lattice thereof.
The superconductor in the partially molten state has a liquid phase. Accordingly, when the supporting methods of the prior art are used, problems are thought to occur such that deformation of a lower portion of a precursor in the partially molten state is caused due to its weight when the precursor supported by several rod-like support members is softened, or the bottom of the precursor adheres to the support member as a result of a reaction between the precursor and the support member, or the like. In addition, in the methods in which an oxide superconductor is supported by a support member or mixed powder which has a composition similar to that of the oxide superconductor, a solidified portion (portions in which a reaction has proceeded spontaneously) forms easily that consists of a composite oxide having a composition that contains rare earth elements, and a subtle difference in the coefficients of thermal expansion between the solidified portion and an oxide superconductor which is surrounding the solidified portion is easily formed. In actuality, the inventors of the present invention obtained experimental results demonstrating that cracks form easily in the vicinity of the interface between the aforementioned solidified portion composed from composite oxide (portions in which a reaction has proceeded spontaneously) and the oxide superconductor portion which is surrounding the solidified portion, when this type of oxide superconductor was attempted to be produced by a partial melting and solidification method wherein solidification is conducted from a partially molten state. Here, in the present invention, “partially molten” state means a state in which the 123 phase is melted, while the 211 phase remains in the solid phase and is dispersed in the molten 123 phase.
Among the various types of oxide superconductors, RE-Ba—Cu—O based oxide superconductors (where RE contains a rare earth element) have a high critical temperature and are widely known.
In this type of oxide superconductors, an oxide superconductor wherein Y is used as a rare earth element thereof and the composition thereof is YBa2Cu3O7-x is considered that it has a low risk of crack propagation throughout entire bulk of the oxide superconductor, even if fine cracks are partially formed, when the bulk of the oxide superconductor is produced by a partial melting and solidification method. In actuality, a bulk of a Y—Ba—Cu—O based superconductor has been produced having a high critical current density and a diameter of about 100 mm.
However, for example, in the case of an oxide superconductor in which Nd is used as the rare earth element, when fine cracks are formed, the cracks easily propagate throughout the oxide superconductor. Accordingly, if partial cracks form in the oxide superconductor, the cracks propagate throughout the oxide superconductor and break the oxide superconductor, or the oxide superconductor tends to have remarkably low superconductivity characteristics and large cracks penetrating through the entire superconductor. For example, when a Nd based bulk having superior superconductivity characteristics is produced using the partial melting and solidification method, a current production limit is a production of a bulk having a diameter of about 30 mm. However, such a bulk having the diameter is unable to be produced at satisfactory yield due to the presence of cracks.
It is understood from documents describe below regarding the difficulty in obtaining large, bulk-shaped oxide superconductors that are free of cracks and other defects, that “As the size of bulk materials made to undergo crystal growth becomes larger, the length of time required for crystal growth increases. As a result, prolonged heat treatment is conducted in the partial molten state, and compositional variations and the like are caused due to a loss in liquid phase components, or contamination caused from materials contained in the substrate or the like, thereby making it difficult to obtain high-quality crystals.” Examples of these documents include a document published by D. A. Cardwell entitled “Processing, microstructure and characterization of artificial joint in top seeded melt grown Y—Ba—Cu—O” in “Institute of Physics Publishing Superconductor Science and Technology, 15 (2002) 639-647”, a document published by Lihua Chen entitled “Joining of melt-textured YBCO: A direct contact method” in “Institute of Physics Publishing Superconductor Science and Technology, 15 (2002) 672-674”, and a document described by Naomichi Sakai, et al. entitled “Microgravity Superconductor Production Project” in “Cryogenics, 34, 11 (1999) p. 563”.
In consideration of the aforementioned problems, an object of the present invention is to provide a technology which enables a production of a large, bulk-shaped oxide superconductor that is free of defects, wherein cracks which are caused by a difference in the coefficients of thermal expansion between an oxide superconductor and a support member are not formed when a production of an oxide superconductor is carried out using a partial melting and solidification methods.
Another object of the present invention is to provide a preferable substrate material for supporting a precursor of an oxide superconductor, which is usable for producing a large, bulk-shaped oxide superconductor free from cracks by using a partial melting and solidification method.
In addition, another object of the present invention is to provide a large, bulk-shaped oxide superconductor free from cracks by using a partial melting and solidification method.
Moreover, another object of the present invention is to provide a technology that enables a production of a bulk-shaped Nd based oxide superconductor having a diameter of about 30 mm or more, which is currently the largest size in the world.